The lipopolysaccharide (LPS) of Chromatium vinosum has anticomplementary activity. This anticomplementary activity is destroyed by alkaline digestion of the LPS and is suppressed by both Mg2+ and Ca2+ ions. Treatment of the LPS with ethylenediaminetetraacetic acid, sodium deoxycholate, or dimethyl sulfoxide did not affect its toxicity toward mice; however, alkaline-treated LPS was not toxic. Treatment of the LPS with sodium deoxycholate, dimethyl sulfoxide, or sodium dodecyl sulfate resulted in reversible dissociation into subunits. Aggregation of the subunits into the original form was achieved by removing the dispersing agent by dialysis against distilled water followed by freezing and thawing. Electron micrographs of phenol-extracted LPS showed long filaments. Electron micrographs of sodium deoxycholate- and sodium dodecyl sulfate-treated and dialyzed LPS showed a mixture of small subunits and short filaments, whereas dimethyl sulfoxide-treated and dialyzed LPS contained only small ovoid spheres. The LPS produced an ordered series of multiple bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A similar banding pattern was observed for Salmonella abortus-equi and Proteus mirabilis LPS. The C. vinosum LPS appears to be mitogenic for mouse spleen cells.